Recently discussions are triggered in 3GPP standardization body to use IP transport bearer for the A-interface (A over IP, AoIP), the interface between the Radio Network (RAN) and the Core Network (CN). In the discussion it is indicated that IP transport on the A-interface user plane connection offers the opportunity to move transcoder functionality from the respective GERAN (GSM EDGE Radio Access Network) node where it is placed within an associated Transcoder and Rate Adapter Unit (TRAU), to a node within the core network, i.e. a Media Gateway (MGW) node.
If transcoder functionality is placed in GERAN according to current 3GPP standard, then PCM is used as codec type on A-interface user plane connection. If transcoder functionality is placed in the core network (option with A over IP) then the same codec type may be used on the air interface (GERAN) and on the A-interface user plane connection.
In a recent, yet unpublished, patent application PCT/EP2007/060586 a procedure is proposed how a BSC (Base Station Controller) may inform a MSC (Mobile Switching Center) about restriction of transcoder resources. In case a BSC does not provide specific transcoder resources for some codec types or specific transcoder resources shall not be used then the codec types are marked (or removed) from the early messages exchanged between BSC and MSC at call setup.
Typically, within the process of channel assignment a MSC sends a list of codec types (e.g. SCVL) applicable for the call to a BSC. The list may be ordered by preference, i.e. in increasing or decreasing preference. The final decision, which codec is used for the channel assignment is done within the BSC. The BSC might select any codec from the list based on internal decision algorithms.
It may be that not all transcoder functionality is moved from the GERAN towards the core network.
In a first Example, it may happen that transcoding is offered in core network for some specific codec type and in GERAN for the same codec type.
In another Example, it may happen that an operator wants to (re-)use its current investment in GERAN (e.g. transcoder for codec type X in a Transcoder and Rate Adapter Unit (TRAU)) and uses the introduction of A over IP (AoIP) to invest in new devices within the core network (transcoder for the codec type Y in MGW).
It is a drawback of the prior art, that the BSC does not have any knowledge of Media Gateway capabilities in general and their availability in particular. Thereby, a BSC is not able to place a transcoding resource within the Core Network without hazarding the consequence that said transcoding resource might be in fact unavailable in the Core Network.
It would therefore be advantageous to have a so far unknown solution allowing for an intelligent decision on and the respective placement of the transcoding functionality within the RAN or within the Core Network, preferably on a call-by-call basis.